1,865 research outputs found
Strong gravitational lensing by wormholes
We study strong gravitational lensing by a class of static wormhole
geometries. Analytical approaches to the same are developed, and the results
differ substantially from strong lensing by black holes, first reported by
Bozza. We consider two distinct situations, one in which the observer and the
source are on the same side of the wormhole throat, and the other in which they
are on opposite sides. Distinctive features in our study arise from the fact
that photon and antiphoton spheres might be present on both sides of the
wormhole throat, and that the throat might itself act as a photon sphere. We
show that strong gravitational lensing thus opens up a rich variety of
possibilities of relativistic image formation, some of which are novel, and are
qualitatively distinct from black hole lensing. These can serve as clear
indicators of exotic wormhole geometries.Comment: 29 pages, 7 figures, published in JCA
Why isn’t India a major global player? : the political economy of trade liberalization
The policy reforms initiated in India in the mid-1980s and expanded in 1991 helped support an expansion in India’s trade. Trade reforms since the mid-1990s have been piecemeal. This paper argues that without significant further reform and adoption of a focused trade strategy, the competitiveness of India’s industry will suffer, including in areas such as information technology and related services in which India has established a strong global niche. Critical building blocks of such strategic reforms include further reductions in tariffs, opening services sectors to foreign competition, serious initiatives to reduce trade transaction costs that prioritize integration into international supply-chains, and a greater focus on regional integration
Modelling Einstein cluster using Einasto profile
We demonstrate a general relativistic approach to model dark matter halos
using the Einstein cluster, with the matter stress-energy generated by
collisionless particles moving on circular geodesics in all possible angular
directions and orbital radii. Such matter, as is known, allows an anisotropic
pressure profile with non-zero tangential but zero radial pressure. We use the
Einasto density profile for the Einstein cluster. Analytical studies on its
properties (metric functions) and stability issues are investigated. Further,
to establish this model (with the Einasto profile) as one for a dark matter
halo, we use the SPARC galactic rotation curve data and estimate the best-fit
values for the model parameters. General relativistic features (beyond the
Keplerian velocities) such as the tangential pressure profile, are
quantitatively explored. Thus, Einstein clusters with the Einasto profile,
which tally well with observations, may be considered as a viable model for
dark matter halos.Comment: 18 pages, 6 figure
Tidal disruption of white dwarfs in a modified gravity theory with SPH
Low energy imprints of modifications to general relativity are often found in
pressure balance equations inside stars. These modifications are then amenable
to tests via astrophysical phenomena, using observational effects in stellar
astrophysics that crucially depend on such equations. One such effect is tidal
disruption of stars in the vicinity of black holes. In this paper, using a
numerical scheme modelled with smoothed particle hydrodynamics, we study real
time tidal disruption of a class of white dwarfs by intermediate-mass black
holes, in the low energy limit of a theory of modified gravity that alters the
internal physics of white dwarfs, namely the Eddington inspired Born-Infeld
theory. In this single parameter extension of general relativity, the
mass-radius relation of white dwarfs as well as their tidal disruption radius
depend on the modified gravity parameter, and these capture the effect of
modifications to general relativity. Our numerical simulations incorporating
these show that departure from general relativity in these scenarios might be
observationally significant, and should therefore be contrasted with data. In
particular, we study observationally relevant physical quantities, i.e., tidal
kick velocity and trajectory deviation of the remnant core and fallback rates
of the tidal debris in this theory and compare them to the Newtonian limit of
general relativity. We also comment on the qualitative differences between the
modified gravity theory and one with stellar rotation.Comment: 23 Page
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